That's how the install(1) program on NetBSD informs you that it doesn't accept long options (options that begin with --).

Besides, the --strip-program=strip flag is useless on the BSDs. That's almost certainly a Solaris or AIX (maybe HP-UX) compat thing. You should just delete it in your Makefile (probably more accurately in CMakeLists.txt).

I'm rather surprised that you say this works on OpenBSD. It shouldn't. OpenBSD's install(1) also lacks longopt support. I don't know about FreeBSD. I wonder if CMake checks for FreeBSD and OpenBSD and removes the flag. Then you should add NetBSD to that check.

Anyhow, simply remove the --strip-program=strip part of the command from your Makefile and it'll work.

No, this port wasn't for me and I'm not a user. Long story:
Someone from another forum has asked how could he build this. I made the suggestion that a port should be easy, so i made only the package recipe. Now you can find this package in pkgsrc-wip branch. I need more testers and cleanups. Maybe someone knows how can i get my pull request integrated into master branch of mosquitto and is Dragonfly BSD already supported?